1. Field of the Invention
The present invention relates to an image display device which allows a stereoscopic image to be observed without special eyeglasses.
2. Description of the Related Art
A device which is known as a prior art of a device for displaying a stereoscopic image without special eyeglasses has a parallax barrier, a lenticular lens or alike (spectroscopic means) situated on an observer side of a display device such as a liquid crystal panel or a plasma display panel (PDP). The spectroscopic means horizontally separates light from left and right eye images, which are displayed on a display panel, to provide a stereoscopic image.
FIG. 52 shows principles of an eyeglass-free stereoscopic image display device which uses a parallax barrier. In the drawing, reference numeral 1 denotes an image display panel and 2 denotes a parallax barrier. Rows, in which left eye pixels L are aligned vertically, and rows, in which right eye pixels R are aligned vertically, are alternately formed on the image display panel (“Autostereoscopic 3D Displays using Image-Splitter Method”, Journal of The Institute of Image Information and Television Engineers, Vol. 51, No. 7, pp. 1070-1078, (1997)). Many slit openings 2a, which extend vertically, are formed on the parallax barrier 2. Barrier portions 2b, which extend vertically, are formed between the openings 2a. It should be noted that there is sufficient binocular parallax between a left eye image, which is formed by the left eye pixels L, and a right eye image, which is formed by the right eye pixels R, for a person to perceive a stereoscopic image. An observer attempting to observe a stereoscopic image may position the head at a predetermined position (ordinary viewing position) so that a left eye image 3L enters the left eye 4L through the opening 2a while a right eye image 3R enters the right eye 4R through the opening 2a to perceive a stereoscopic image. Meanwhile, light of the right eye image is blocked by the barrier portion 2b and prevented from entering the left eye 4L, and light of the left eye image is blocked by the barrier portion 2b and prevented from entering the right eye 4R. However, it has been figured out so far that such a stereoscopic image display device causes interference fringes (moiré) between a pattern of the parallax barrier and a pixel pattern of a plasma display, and that a moiré condition depends on a width or a shape of the openings of the parallax barrier. In general, a region referred to as a black matrix for preventing color mixture among RGB sub-pixels exists in a liquid crystal display (LCD) or a PDP (a black matrix is also referred to as a rib for LCD). Besides the black matrix between sub-pixels, auxiliary electrodes or alike may be arranged on each sub-pixel. Therefore, the black matrix and the auxiliary electrodes are observed through slits in the parallax barrier. Consequently, contrast is created between an opening, in which there is a large visible ratio of the black matrix and the auxiliary electrode, and another opening, in which there is a small visible ratio of the black matrix and the auxiliary electrode although it depends on observing positions. Accordingly, uneven luminance (moiré) is created on a screen and causes problems about significant degrade of image quality. As an example of moiré, FIG. 48 shows moiré patterns observed through a step barrier, which has stepped slits in front of a display displaying white on the entire display screen, and through a slanted barrier, which has slanted slits in front of the display. In this case, the opened slits in the drawing is horizontally as wide as sub-pixels (opening ratio of 1). As shown in the drawing, in the case of a step barrier, grid-like moiré is likely to result from fluctuation in a mixture ratio between black matrix portions and pixel portions of upper, lower, left and right regions which are visible through slits of the step barrier. The fluctuation is caused by a change in observing position. In contrast, there is a smaller fluctuation of an observed pixel area in the case of a slanted barrier than in the case of a step barrier, regardless of a positional relationship. Therefore, the slanted barrier is likely to cause weaker contrast of moiré than the step barrier. In particular, horizontal moiré patterns become less visible. However, in both cases, moiré patterns are visible in this manner. Notable image deterioration occurs during 2D viewing but not 3D viewing. In order to remove such moiré patterns in 3D image display, a method, in which a first plate having a pattern formed in a first cycle and a second plate having a pattern formed in a second cycle are crossed at a predetermined angle, has already been proposed. FIG. 49 schematically shows this condition. FIG. 49 shows a barrier pattern inclined at a range of 20 to 30 degrees with respect to pixels in order to reduce moiré (US 2005-0073472).
Another prior art example, in which a vertical stripe pattern in a tooth shape with a half size of a barrier pitch, has been reported, as shown in FIG. 50. In this case, averaging between pixels and a black matrix increases (U.S. Pat. No. 7,268,943). Besides the aforementioned shape, an example using a zigzag or curved pattern as shown in FIG. 51 has also been reported (WO 2010/007787).
US Patent Application No. 2005-0073472 describes a method for making moiré less noticeable by significantly inclining a barrier as shown in FIG. 49 to increase degree of suppressing a fluctuation in observed pixel area from an observing position. However, when actually inclining a barrier angle, adjacent pixels become more visible from a single slot simultaneously. Accordingly, crosstalk increases.
With a vertical stripe pattern in tooth shape which has a half size of a barrier pitch as shown in FIG. 50, an increase in average opening ratio makes image blur more noticeable because of an increase in crosstalk although averaging between pixels and a black matrix increases as described in U.S. Pat. No. 7,268,943. As described below, in the cause of such a size, adverse effects are more likely to result from the tooth shape itself because of a balance between a size and a number of viewpoints.
As described in WO 2010/007787, cases using a zigzag or a curved pattern as shown in FIG. 51 have also been reported. These cases aim to make jump points less noticeable by resultant mixture between adjacent parallax pixels from elliptical arc edges of openings. In other words, like the two conventional examples described above, image blur is likely to become more noticeable because of an increase in crosstalk.
In the aforementioned circumference, a reduction in moiré contrast is inadvertently accompanied by an increase in crosstalk. In other words, there is a tradeoff between moiré intensity and an amount of crosstalk, and improving one worsens the other. Therefore, a challenge is to determine what kind of measure is capable of reducing moiré intensity with an insignificant increase in crosstalk.